The present invention is described in connection with controlling the rotational speed of a driving motor for a magnetic disc memory. However, it is to be understood that the invention is, in certain respects, applicable to driving any electric motor having a high velocity output shaft wherein the velocity must be controlled with very great precision, for example greater than 10.sup.-4.
Magnetic disc memories are extensively utilized in data processing systems because such memories have extremely high storage capacity and require relatively short access time to read signals from and write signals into the storage medium by way of read and/or write heads for transducing information between the disc and readout circuitry. Magnetic discs carry data on circular, concentric recording tracks having radial widths typically no greater than a few hundredths of a millimeter. The recording tracks cover a relatively high proportion of both surfaces of the disc. In presently available discs, in particular discs for memories having a relatively small number of discs, such as fewer than four or five discs, data are contained on each disc surface and distributed along equal and adjacent circular sectors. One disc surface is normally divided into several decades of sectors, such as forty to fifty sectors. A detailed description of such a data distribution topography is found, for example, in copending U.S. application (LKP&B docket 037-069), Ser. No. 186,294, filed Sept. 11, 1980, entitled Method of and Apparatus for Displacing a Movable System With Respect to a Data Carrier, and commonly assigned with the present invention. The magnetic discs of a disc memory are coaxially mounted in parallel planes and usually of identical diameter. The several discs forming a disc memory are driven by an output shaft of a single DC electric motor.
It is presently the trend in the development of magnetic discs to increase the longitudinal, i.e., linear, density of information packed in a unit length along the circumference of a track on the disc. The mean number of binary information items contained in a magnetic disc track may, at present, reach one hundred thousand. Because of this high packing density, it is necessary to ensure that the data are written into or read from the disc with a minimum error risk.
To this end, it is necessary to determine the time the beginning of each sector passes beneath a magnetic read/write head with great precision. The beginning of a sector is the part of a sector which is chronologically the first portion of the sector to pass beneath a read/write head for transducing magnetic data on a disc surface into electric signals. The beginning of a sector is assumed to coincide with a diameter of the disc surface. If m is the number of sectors on a disc surface, any sector of rank k amongst the m sectors is chronologically positioned from a given reference time by providing, at the periphery of the disc, beyond the recording tracks containing the binary data, a special, reference magnetic datum, commonly referred to as an "index". The housing of the magnetic disc memory carries a magnetic readout transducer which is located so that during each turn of the disc, the magnetic datum index passes beneath the transducer at a particular time t.sub.0. The magnetic read transducer derives an analog electric signal pulse in response to the index pasing beneath it. A shaping circuit converts this electric pulse into a relatively short duration, precisely timed pulse that is part of a binary or logic signal. It is to be recalled that an analog signal is a signal having a voltage or current amplitude that continuously varies between a pair of limits, one of which is usually positive and the other of which is usually negative. A logic or binary signal is a signal which can assume no more than two values, referred to as binary or logic zero or binary or logic one levels.
In the prior art, the location of any sector of rank k of the disc is determined by measuring the time which elapses between time t.sub.0, when the index mark is beneath the transducer, and the time t.sub.k when a read head associated with the disc derives a pulse corresponding to the beginning of the sector of rank k. Because the rotation speed of the disc is assumed to be perfectly constant, the interval separating times t.sub.0 and t.sub.k is accurately known and predetermined. To locate the start of an optional disc sector with high precision, it is necessary for the interval t.sub.k -t.sub.0 to always be equal in time to the quantity .DELTA.t, where .DELTA.t is an incremental time that is the same for each sector of rank k from time t.sub.0. This is true regardless of the time at which it is sought to locate the beginning of the sector of rank k. It follows that the rotational speed of the motor driving the disc must remain exactly constant as a function of time, i.e., the disc drive motor speed must be controlled with very great precision. In situations in which the linear data packing density on the disc is on the order of one hundred thousand information items per track, it is mandatory for the number of erroneous data read from or written into the track not to exceed one unit, i.e., a read or write precision of 10.sup.-5 is necessary. In these circumstances, it is necessary for the rotational speed of the disc drive motor to be controlled with a precision at least equal to 10.sup.-5.
To control the motor speed to this precision, the true rotational speed v.sub.m of the motor is measured and compared against a reference speed v.sub.R, to derive a speed error signal .DELTA.v=(v.sub.m -v.sub.R). The voltage or current supplied to an armature or field winding of the DC drive motor for the disc memory, and therefore the motor speed, is controlled as a function of the error signal .DELTA.v.
The prior art device for carrying out the motor speed control includes an initializing means formed by an index transducer responsive to the index on the periphery of the disc. The index transducer derives a pulse in response to the disc index passing beneath it. The pulse derived by the transducer is supplied to a circuit for shaping the pulse into a relatively narrow, precisely defined pulse having a leading or trailing edge with a predetermined time position relative to the index passing the index transducer. During each turn of the disc, the initializing means thus derives a relatively narrow logic pulse having a leading or trailing edge which defines the initial time t.sub.0 for a reference position of the disc during each turn thereof. If i denotes the i-th revolution of the disc and (i+1) denotes the (i+l) th revolution of the disc, the corresponding pulses are derived at times specified by t.sub.0i and t.sub.(i+1).
The prior art device also includes timing means for deriving a periodic, rectangular pulse signal having a period P and frequency F, such that the frequency and period of the signal are highly accurate. A speed measuring means responds to the signals derived by the initializing means and the timing means, to determine the disc speed v.sub.m. Disc speed v.sub.m is determined by measuring the number of rectangular pulses derived by the timing means during the interval between times t.sub.0i and t.sub.0(i+l). The number of rectangular pulses derived during the interval from t.sub.0 to t.sub.0(i+1) is expressed in analog form as a variable amplitude signal current or voltage, or in digital form as a number of logic pulses.
A comparing means responds to the determined speed v.sub.m and a reference speed v.sub.R to derive an analog error signal having a magnitude proportional to .DELTA.v. A voltage or current source for the motor is responsive to the analog voltage or current proportional to .DELTA.v, to supply the armature or field winding of the motor with a voltage or current having a magnitude proportional to .DELTA.v. The prior art device employs speed measuring and comparison means that are formed by different elements containing a significant number of analog circuits. The extensive use of analog circuits, particularly for the comparator, impairs the precision with which the velocity of the motor speed can be controlled and frequently makes it impossible to achieve control of the disc speed to a precision of at least 10.sup.-5.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for controlling the rotational velocity of a motor.
Another object of the invention is to provide a new and improved apparatus for precisely controlling the rotational velocity of a motor driving a magnetic storage disc.
A further object of the invention is to provide a new and improved apparatus for controlling the rotational speed of a motor shaft with a precision of at least 10.sup.-5.
An additional object of the invention is to provide a new and improved apparatus for controlling the speed of a magnetic disc containing on the order of ten thousand information items per track, such that the number of erroneous data read from a track of the disc does not exceed one unit.
A further object of the invention is to provide a new and improved apparatus for precisely controlling the rotational speed of a motor by employing a digital circuit for deriving an error signal for the motor speed.